This invention relates to a limp, porous membrane of sheet or cloth material for a fluidized outlet. More specifically, this invention relates to the fluidized outlet of a covered hopper railway car.
Generally, when unloading a bulk pulverant, powdered, or particulate lading from a hopper, such as a hopper of a covered hopper car, such ladings have a tendency to bridge and column inside the hopper. In order to continue the unloading, it is a common practice to vibrate or hammer on the outside of the hopper, thereby to dislodge the bridged and columned lading. It is also conventional to use probes inserted into the lading from above the break up the bridges and columns within the lading.
It has been known that by using a fluidized outlet built into the bottom of the hopper, fluidizing fluid (e.g., air under relatively low pressure, 5-15 psi) flowing through a fluidizing, porous membrane and into the powdered or particulate lading :n contact with the membrane would cause the lading to be fluidized so that the lading would flow as freely as a liquid. This resulted in faster and much more efficient unloading of the railway hopper car.
As shown in the co-assigned U.S. Pat. Nos. 4,280,706 and 4,568,244, prior art fluidizing membranes conventionally comprised a porous screen or the like made of fine mesh stainless steel. However, while stainless steel fluidizing membranes were relatively impervious to moisture pick-up, could be readily cleaned, and were long-lasting, they were relatively expensive, both to install and to replace if necessary. On a larger size of outlets where the outlet had both side and end walls converging inwardly and downwardly from the central lower outlet opening, the stainless steel membrane was often formed in one piece so as to be disposed on the inside faces of the end and side walls. During unloading, particularly upon exhausting of all or substantially all of the lading from within the hopper, the stainless steel mesh, which was relatively rigid, had sufficient force to withstand the static air pressure behind the porous membrane in the outlet walls without damage to the porous membrane.
It has long been recognized that limp fabric membranes could be used in place of the stainless steel membranes with large attendant cost savings. However, cloth membranes were relatively difficult to seal relative at the connection of the cloth membrane and the outlet frame and to the outlet opening at the bottom proximate the outlet opening. In addition, cloth membranes oftentimes did not have sufficient strength to withstand the static air pressure therebehind when the lading was nearly withdrawn from the outlet. It will be appreciated that the static air pressure would tend to "balloon" the membrane material away from the walls of the outlet when the hopper was substantially free of the lading thus putting undue tearing forces on the membrane where it joined the outlet frame and the bottom outlet. As a consequence, oftentimes, fabric membranes were only used in relatively small panels in which the cloth membrane could be secured around its periphery to a rigid metal frame, such as shown in the co-assigned U.S. Pat. No. 4,280,706.
Thus, there has been a long-standing need for a relatively inexpensive, limpcloth fluidizing membrane which could be used on large fluidized outlet walls and surfaces which did not experience undue "ballooning" when pressurized such that the membrane would remain attached to the outlet, both at the outlet frame and at the outlet opening.